Polyphenols (noun, pronunciation of the singular or , also known as polyhydroxyphenols) are a structural class of mainly natural, but also synthetic or semisynthetic, organic chemicals characterized by the presence of large multiples of phenol structural units. The number and characteristics of these phenol structures underlie the unique physical, chemical, and biological (metabolic, toxic, therapeutic, etc.) properties of particular members of the class. Examples include tannic acid (image at right), and ellagitannin (image below). The historically important chemical class of tannins is a subset of the polyphenols.